Abstract: FeOnanoparticles were fabricated by the chemical coprecipitation by using Triton X-100 34 as dispersant. Hollow Polyacronitrile (PAN)/FeOmagnetic composite nanofibers were fabricated 34

through coaxial electrospinning and post-treatment. The effect of sheath feed rate on the formation of hollow structure was investigated and hollow structures of composite nanofibers were characterized

Microwave absorption materials have attracted a great deal of attention due to their potential applications in wireless data communication, satellite television and military facilities in recent years. Magnetite as a conventional microwave absorption material has played an important role in the development of microwave absorption materials for its high specific resistance and excellent

such as magnetic metal and ferrite, are too heavy to meet specific applications in many fields. [3]Coupling with low density substrates can solve this problem .

A lot of research work has focused on polymer-based composites filled with magnetic

[4–5]materials in micrometer-size, such as Ba-ferrite, Ni Zn-ferrite and FeO/YIG . However, these 34

30 materials have difficulty in meeting the criterion in thin and light weight microwave absorber and exhibiting a strong reflection to over a wide frequency range. Coaxial electrospinning is a

[6]straightforward technique to prepare polymer fibers with core-sheath or hollow structure . In a

typical process, coaxial electrospinning uses one spinneret which consists of two capillaries coaxially positioned within one another. The effects of electrospinning parameters (for example

35 voltage value) and solution properties such as viscosity, conductivity and surface tension, on the morphology and formation of nanofibers have been extensively studied. The flow rate of the outer and inner solution and the length of the outer and inner capillaries play a leading role in the

[7-8]formation of the core-sheath structure . And magnetite (FeO), agglomerate easily for their 34

magnetic and nano-size, which could affect the performance in its application. Electrospinning

40 technique combines with FeOnanoparticles can reduce the aggregation of nanoparticles. 34

The composites of hollow nanofibers with FeOnonoparticles not only have a lighter weight 34

but also generate new absorbing mechanism to improve the performance of microwave absorption.

Foundations: the Fundamental Research Funds for the Central Universities (No. JUSRP11102 and JUSRP20903); China National Natural Science Foundation (No. 51006046); the Natural Science Fundation of Jiangsu Province (No. BK2010140); the Research Fund for the Doctoral Program of Higher Education of China (No. 200802951011 and 20090093110004).

coaxial electrospinning was PVP / DMF solution with a concentration of 30wt%.

Coaxial electrospinning was performed with varying flow rate of the sheath solution. The 80 flow rate of the core solution was kept at 0.2 mL/h and the flow rate of the core solution varied

from 0.3 mL/h to 0.5 mL/h. The length of the outer nozzle over the inner nozzle was adjusted to

about 0.5 mm. The other conditions of coaxial electrospining are the same as the electrospun

procession of solid PAN/ FeOcomposite nanofibers. 34

The samples of core/shell nanofibers obtained from coaxial electrospinning were immersed in 85 deionized water at 40~50?for 48 h and the water was changed every 12 h. After immersion, the

-2-

豆丁网地址，/msn369

core component PVP was removed, the nanofibers of hollow structure were formed. The hollow nanofibers were vacuum dried for 9 h at 60?. 1.4 CharacterizationDistribution of FeOnanoparticles in fibers was examined using transmission electron 34 microscopy (TEM, JEOL JEM-2100). The morphology of the PAN/FeOnanocomposite fibers 34 90 and the cross-sectional structure of hollow PAN/FeOnanofibers were observed using scanning 34 electron microscopy (SEM, Hitachi S-4800). To fix the hollow structure of nanofibers, the nanofibers were heat treated at 250 ?for 5 min under the heating rate of 3?/min when viewedthe cross-sectional structure of nanofibers. The average fiber diameter of the electrospun nanofibers was measured by using Photoshop 7.0 software. 95

2 Results and discussion2.1 Microstructure of solid PAN/ FeOcomposite nanofibers34 a b c Fig.1 TEM of “a” FeO，“b” PAN/ FeO(92.5:7.5) and “c” PAN/ FeO(90:10)34 34 34 105 TEM observations clearly revealed that structure of the FeOnanoparticles and the 34 dispersion of FeOnanoparticles in PAN nanofibers, as illustrated in Figure 1. The average 34 diameter of FeOnanoparticles was about 26.2 nm which corresponded to XRD result. The FeO34 34 nanoparticles appeared to scatter uniformly inside the fibers and FeOnanoparticles aggregated 34 110

into small clusters in the fiber, as shown in Figures b and c. It was obviously observed that there were more FeOnanoparticles in the fibers as the FeOweight ratio increased. However, FeO34 34 34

nanoparticles in Figure 1c looked less aggregated that that in Figure 1b, which might be attributed to the effect of ultrasonic dispersion and Triton X-100. FeOnanoparticles could agglomerate 34 into large aggregates easily due to high surface force and magnetic force. Ultrasonic vibration 115 [9]could destroy the coulomb force and van der waals force making large aggregates broken .

Figure 3 shows the structures of hollow PAN/FeOnanofibers after the removal of the inner 34 component PVP by immersing in deionized water. The different flow rates of core fluid and shell

fluid significantly affect the structure of hollow PAN/FeOnanofibers, as indicated in Figure 3. 34 When V:V=0.3:0.2, some nanofibers became flat and collapsed. The main reason was the shellcore

140 sheath flow rate was too slow in this case, therefore the sheath solution could not coat the core solution fully during electrospining. The sheath solution was the driving fluid in coaxial electrospinning, if its flow rate was too small, the core solution would not completely follow the

sheath to rotate and move at high speed under high voltage electric field. Flat nanofibers would appear due to the asynchronous motion of core and sheath solution. When V:V=0.4:0.2, shellcore

homogeneous hollow PAN/FeOnanofibers were obtained. When V:V=0.5:0.2, less 145 34 shellcore hollow fiber were formed and more solid fibers were obtained. If the sheath flow rate was too fast, the core flow rate was not fast enough to form the core-shell nanofibers, leading to the formation of solid nanofibers. 2.3 XRD

crystal planes of FeO(PDF #65-3107) respectively. The average grain size (D) was estimated 34 from the Scherrer Equation ,D=κλ！βcosθ (Where K was a constant taken as the normal value of

0.89，λ was 0.154 nm，β is the full width at half-maximum (FWHM), and θ was the Bragg angle).

160 The peak at 2 θ= 35.56? was used to estimate the particle size. The calculated value was about

25.9 nm which was close to the average diameter of FeOnanoparticles characterized by TEM, as 34 shown Figure 1. The crystalline peaks of solid PAN/ FeO(9.1%) composite nanofibers were 34 located at 30.22?, 35.56?, 57.20 ?and 62.74?. The diffraction peaks’ positions of composite nanofibers were the same as those of FeOnanoparticles, however, the intensity of the diffraction 34

peak decreased significantly. These observations indicated that it was physical combination 165

between FeOnanoparticles and PAN without chemical reaction. The crystal construction of 34 FeOnanoparticles in nanofibers did not change after electrospining. 34 2.4 Magnetic propertiesFigure 5 shows the plots of magnetization (M) versus magnetic field (H) of FeO34

nanoparticles and PAN/FeOcomposite nanofibers. The result illustrated that the saturation 170 34 magnetization (Ms), remanent magnetization (Mr) and coercivity(Hc) values of FeO34

nanopartices were 76.9emu/g，6.8emu/g and 51 Oe. However the corresponding values of PAN/FeOcomposite nanofibers were 6.7emu/g, 0.44emu/g and 49 Oe. The FeOnanoparticle loading 34 34 estimated from the Ms was found to be 8.7 wt% (Ms is proportional to the amount of FeO). The 34possible reason was that the magnetic FeOnanoparticles coated by PAN nanocomposite 175 34

hindered the magnetic expression of FeOnanoparticles. It meant that the magnetic properties 34

could be tailored by adjusting the amount of FeOin the nanocomposite. 34

195 composite nanofibers was less than ?10 dB over the range of 8.75–18GHz and 8.1-18 GHz，respectively. The reflection loss of PAN/ FeOand hollow PAN/ FeOcomposite nanofibers was 34 34 less than ?20 dB over the range of 12.7–18GHz and 10.6-18 GHz，respectively. Additionally,

when the reflectivity reached ?10 dB, the reflection loss of the microwave absorption materials achieved 90%. When the reflectivity was ?20 dB the reflection loss of microwave absorption

205 It was found that microwave absorption properties of hollow PAN/FeOcomposite 34 nanofibers looked better than those of PAN/ FeOnanocmposites. PAN/FeOnanofibers can be 34 34

regarded as a type of monolayer microwave-absorbing material, but the hollow PAN/FeO34 nanofibers can be considered as multi-layer microwave-absorbing material with air trapped inside.

When microwave enters the hollow fibers, it goes through three-Rams (Radar absorbing materials)

210 and would be reflected more times in the hollow structure. Multi-layers can expand absorption bandwidth and enhances absorbing effect to a certain extent. Therefore, hollow structure not only

contributes in weight but also improves the performance of microwave absorption. 3 ConclusionHollow PAN/FeOcomposite nanofibers have been fabricated by coaxial electrospinning 34

and post-treatment. It was found that homogeneous hollow PAN/FeOnanofibers could be 215 34 obtained after post-treatment, when V:V=0.4:0.2. XRD patterns indicated that it was physical sellcorecombination between FeOnanoparticles and PAN and the crystal configuration of FeO34 34 nanoparticles remained unchanged after electrospining. Magnetic properties of composite nanofibers could be changed by adjusting the amount of FeOin the nanocomposite. The hollow 34